1. Field of the Invention
The invention relates generally to the deposition of materials and more specifically, to devices, materials and methods for direct writing of a source material onto a substrate using a first laser and then transforming the source material into a material of interest by means of a second laser.
2. Description of the Related Art
The term “direct write” refers generally to any technique for creating a pattern directly on a substrate, either by adding or removing material from the substrate, without the use of a mask or preexisting form. Direct write technologies have been developed in response to a need in the electronics industry for a means to rapidly prototype passive circuit elements on various substrates, especially in the mesoscopic regime, that is, electronic devices that straddle the size range between conventional microelectronics (sub-micron-range) and traditional surface mount components (10+ mm-range). (Direct writing may also be accomplished in the sub-micron range using electron beams or focused ion beams, but these techniques, because of their small scale, are not appropriate for large scale rapid prototyping.) Direct writing allows for circuits to be prototyped without iterations in photolithographic mask design and allows the rapid evaluation of the performance of circuits too difficult to accurately model. Further, direct writing allows for the size of printed circuit boards and other structures to be reduced by allowing passive circuit elements to be conformably incorporated into the structure. Direct writing can be controlled with CAD/CAM programs, thereby allowing electronic circuits to be fabricated by machinery operated by unskilled personnel or allowing designers to move quickly from a design to a working prototype. Mesoscopic direct write technologies have the potential to enable new capabilities to produce next generation applications in the mesoscopic regime. Other applications of direct write technologies in microelectronic fabrication include forming ohmic contacts, forming interconnects for circuit and photolithographic mask repair, device restructuring and customization, design and fault correction.
Currently known direct write technologies for adding materials to a substrate include ink jet printing, Micropen©, laser chemical vapor deposition (LCVD) and laser engineered nano-shaping (LENS). Currently known direct write technologies for removing material from a substrate include laser machining, laser trimming and laser drilling.
The direct writing techniques of ink jet printing, screening and Micropen© are wet techniques, that is, the material to be deposited is combined with a solvent or binder and is squirted onto a substrate. The solvent or binder must later be removed by a drying or curing process, which limits the flexibility and capability of these approaches. In addition, wet techniques are inherently limited by viscoelastic properties of the fluid in which the particles are suspended or dissolved.
In the direct writing technique known as “laser induced forward transfer” (LIFT), a pulsed laser beam is directed through a laser-transparent target substrate to strike a film of material coated on the opposite side of the target substrate. The laser vaporizes the film material as it absorbs the laser radiation and, due to the transfer of momentum, the material is removed from the target substrate and is redeposited on a receiving substrate that is placed in proximity to the target substrate. Laser induced forward transfer is typically used to transfer opaque thin films, typically metals, from a pre-coated laser transparent support, typically glass, SiO2, Al2O3, SrTiO3, etc., to the receiving substrate. Various methods of laser-induced forward transfer are described in, for example, the following U.S. patents and publications incorporated herein by reference: U.S. Pat. No. 4,064,205 to Landsman; U.S. Pat. No. 4,752,455 to Mayer; U.S. Pat. No. 4,895,735 to Cook; U.S. Pat. No. 5,725,706 to Thoma et al; U.S. Pat. No. 5,292,559 to Joyce, Jr. et al; U.S. Pat. No. 5,492,861 to Opower; U.S. Pat. No. 5,725,914 to Opower; U.S. Pat. No. 5,736,464 to Opower; U.S. Pat. No. 4,970,196 to Kim et al; U.S. Pat. No. 5,173,441 to Yu et al; U.S. Pat. No. 4,987,006 to Williams et al; U.S. Pat. No. 5,567,336 to Tatah; U.S. Pat. No. 4,702,958 to Itoh et al; German Patent No. 2113336 to Thomson-CSF and Bohandy et al, “Metal Deposition from a Supported Metal Film Using an Excimer Laser”, J. Appl. Phys. 60 (4) 15 Aug. 1986, pp 1538–1539. Because the film material is vaporized by the action of the laser, laser induced forward transfer is inherently a homogeneous, pyrolytic technique and typically cannot be used to deposit complex crystalline, multi-component materials or materials that have a crystallization temperature well above room temperature because the resulting deposited material will be a weakly adherent amorphous coating. Moreover, because the material to be transferred is vaporized, it becomes more reactive and can more easily become degraded, oxidized or contaminated. The method is not well suited for the transfer of organic materials, since many organic materials are fragile and thermally labile and can be irreversibly damaged during deposition. Moreover, functional groups on an organic polymer can be irreversibly damaged by direct exposure to laser energy. Other disadvantages of the laser induced forward transfer technique include poor uniformity, morphology, adhesion, and resolution. Further, because of the high temperatures involved in the process, there is a danger of ablation or sputtering of the support, which can cause the incorporation of impurities in the material that is deposited on the receiving substrate. Another disadvantage of laser induced forward transfer is that it typically requires that the coating of the material to be transferred be a thin coating, generally less that 1 μm thick. Because of this requirement, it is very time-consuming to transfer more than very small amounts of material.
In a simple variation of the laser induced forward deposition technique, the target substrate is coated with several layers of materials. The outermost layer, that is, the layer closest to the receiving substrate, consists of the material to be deposited and the innermost layer consists of a material that absorbs laser energy and becomes vaporized, causing the outermost layer to be propelled against the receiving substrate. Variations of this technique are described in, for example, the following U.S. patents and publications incorporated herein by reference: U.S. Pat. No. 5,171,650 to Ellis et al, U.S. Pat. No. 5,256,506 to Ellis et al, U.S. Pat. No. 4,987,006 to Williams et al, U.S. Pat. No. 5,156,938 to Foley et al and Tolbert et al, “Laser Ablation Transfer Imaging Using Picosecond Optical pulses: Ultra-High Speed, Lower Threshold and High Resolution” Journal of Imaging Science and Technology, Vol. 37, No. 5, September/October 1993pp. 485–489. A disadvantage of this method is that, because of the multiple layers, it is difficult or impossible to achieve the high degree of homogeneity of deposited material on the receiving substrate required, for example, for the construction of electronic devices, sensing devices or passivation coatings.
The direct write technique called laser chemical vapor deposition (LCVD) utilizes a laser beam focused on the surface of a substrate to induce localized chemical reactions. Usually the surface of the substrate is coated with a metal-organic precursor, which is either pyrolyzed or photolyzed locally where the laser beam scans. Pyrolytic laser CVD involves essentially the same mechanism and chemistry as conventional thermal CVD, and it has found major use in direct writing of thin films for semiconductor applications. In photolytic CVD, the chemical reaction is induced by the interaction between the laser light and the precursors. A limitation of both processes is that they must be carried out under controlled atmospheres such as inside a vacuum system and they tend to exhibit slow deposition rates. In addition, they are not well suited for direct write applications where multilayers of dissimilar materials need to be produced.
The direct write technique called LENS, utilizes a laser beam to melt powders of various materials as they come in contact with the substrate surface. LENS is a process that works well for making thick layers of metals, however, the high melting points exhibited by most ceramics required the use of high power laser beams that cause overheating of the substrate and surrounding layers. Furthermore, many ceramics once melted will not exhibit their original crystalline structure after solidification. In addition, because the materials being deposited must first melt and then resolidify, the layers are under large stresses which can cause their delamination.
A direct write method of laser transfer of certain types of materials is described in U.S. patent application Ser. No. 09/318,134 for “MATRIX ASSISTED PULSED LASER EVAPORATION DIRECT WRITE” filed on May 25, 1999 by Chrisey et al.
There are some materials that are easier to transfer in a precursor state or as a mixture of a precursor and a powdered form, but when they are transferred in this form, they do not have desired physical properties such as electrical conductance. In order to optimize desired qualities such as electrical conductance, further processing of the materials is necessary. Therefore, there is a strong need for devices and methods for transferring materials in a precursor form under ambient conditions (that is, atmospheric pressure and room temperature), and then transforming the precursor into a more useful or desirable form.